Dry rectifier element and method of manufacture



.; 1948. c. A. KOTTERMAN ,452,77

DRY RECTIFIER ELEMENT AND METHOD OF MANUFACTURE Filed Oct. 7, 1945 T,WWW I IlllH 2 V77 J Wfl/ ////J INVENTOR. 67/557257? 4. AOTIf/F'AMPatented Nov. 2, 1948 DRY RECTIFIER ELEMENT AND METHOD OF MANUFACTUREChester A. Kotterman, Livingston. N. J assignor to Federal Telephone andRadio Corporation, New York, N. Y., a corporation of DelawareApplication October 7, 1943, Serial No. 505,330

4 Claims.

er is applied by sprinkling a suitable material in powder form on thebase plate and compressing it thereon at an elevated temperature wherebythe powder softens sufllciently to form a uniform layer and adhere tothe plate. When the semi-conducting material is selenium, pressures ofthree to five thousand pounds per square inch and temperatures of 110 C.to 135 C. are satisfactory. The selenium coated base plate may then befurther heat treated to obtain the required crystal form of the seleniumand is further processedto adapt it for use as a rectifier.Specifically, a, counter-electrode is applied over the surface, of theselenium, as by spraying with a suitable conducting substance such asWoods metal, and the element may then be given an electro-formingtreatment by the application of voltage between the base plate and thecounterelectrode.

It has been found in using such rectifier elements that the optimumthickness for the semiconducting layer is within a definite range,usually of the order of .003 to .006 inch. When thinner layers are usedthe rectifyi g action b'eaks down while when thicker layers are used therectifiers offer too much resistance. to the passage of current,resulting decreased efficiency and heating of the rectifiers to a pointtoo near their maximum safe operating temperature, which is about 75 C.In prior practice, where ordinary fiat discs are used for the baseplates, the value of the pressure applied has been critical indetermining the thickness of the layers produced, since, assuming anexcess of powder is used, too little pressure results in insufficientadhesion and too much pressure tends to extrude too much of the softenedmaterial laterally. It therefore has been difiicult to calculate theexact pressure required to produce the optimum thickness.

The present invention produces in a simple manner dry rectifier elementswith semi-conducting layers of specific, predetermined thickness byusing base plates having upstanding circumferential rims. The height ofthe rim determines the thickness of the layer, since the upper 2 pressplaten used in the pressing Operation is substantially larger than theelement and is arrested by the rim; so that regardless of the pressureapplied, there will be no thinning out of the selenium layer beyond thedesired limit. By making use of a suitable punch press for punching "outthe base plates, plates with rims of the desired height can be obtainedin one simple operation from which in turn rectifiers with semiconducting layers of a thickness corresponding to the height of saidrims can be produced by standard methods. A corollary object oftheinvene tion is to eliminate the necessity for highly accuratemeasurement of the quantity of powder used, and at the same time toobtain a semiconducting layer of specific thickness. A further object ofthe invention is to reduce-the occurrence of chipped edges in saidlayer.

Other objects and advantages will appear in the following detaileddescription in connection with the attached drawings in which:

Fig. 1 is a face view and Fig. 2 a. cross section of a base plate with araised circumferential rim, the height of the rim being exaggerated;

Fig. 3 is an expanded view of the press platens and an interposed baseplate shown in section and carrying the selenium powder; and

Fig. 4 is a cross section of a completed rectifier made according to theinvention.

Figs. 1 and 2 show a base plate of a type readily punched out by asuitable punch press, having a raised rim l around the edge, withrespect to which the surface 2 of the plate 3 is depressed. Theelevation of the rim is preferably about .004

- inch, representing the desired thickness of the selenium layer, butmay be more or less than this, and of course is exaggerated in thedrawings to make the construction clear. The width of the rim may vary,one embodiment having a rim Width of about one sixteenth of an inch. Theplate will ordinarily have a central opening a as shown so that a numberof rectifiers may be mounted in known manner with suitable spacingmembers interposed on an insulated spindle running through the openingsto form a stack,

Figure 3 illustrates the method of forming the selenium layer. A layerof selenium powder 5 is applied to the surface 2 of the base plate 3 asby sifting it in place; and an excess of powder is advantageously 'usedto assure the continuity and uniform thickness of the selenium layerthroughout said surface. Platens 6 are heated to a temperature at whichthe powder 5 will soften and fuse; and when said platens are broughttogether with the base plate 3 carrying 3 the powder 5 located betweenthem, the application of heat and pressure will change the powder 5 to afluent mass which will be forced into efi'ective engagement with surface2, the surplus material flowing out over the rim I and into the aperture4 where it is readily removed. The pressures and temperatures employedare known to those skilled in the art and are the same to those whichare used in the ordinary pressed powder method of applying the seleniumlayer, except that substantially higher pressures can be employedwithout damage because of the use of the rim I. Such higher pressuresare advantageous because they produce substantially better adhesion tothe surface 2, as well as greater assurance of uniformity in thicknessof the selenium layer.

The unit is then treated in the usual manner to form the finished disk.This treatment normally includes a second heat treatment, ordinarily ata temperature of about 200 C., to transform the selenium into thedesired crystalline form. The counter electrode is thenapplied tothickness and a-flat surface extending inwardly from the rim; disposingon said surface a quantity of a thermoplastic semi-conducting material,

and forming said material into a flat layer flush with the upper marginof the rim by pressing a flat hot surface against the material andforcing said hot surface toward the base plate until arrested by therim, the heat and pressure being sufllcient to fuse the material andform a continuous uniform layer strongly adherent to the base platesurface.

2. A process as set forth in claim 1 in which an excess of thesemi-conducting material is disposed on said surface, the excessmaterial being laterally extruded upon the application of heat andpressure.

3. A process asset forth in claim 1 in which the semi-conductingmaterial is in the form of a the selenium as by spraying, and the diskis ment having a semi-conducting layer of specific thickness, comprisingforming a base plate with an upstanding rim having a height equal tosaid powder.

4. A process as set forth in claim 1 in which the semi-conductingmaterial is selenium.

CHESTER A. KO'ITERMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,611,653 Lilienfeld Dec. 21,1926 1,826,955 Ruben Oct. 13, 1931 1,954,950 Russell Apr. 17, 19341,989,463 Ruben Jan. 29, 1935 2,032,439 Ruben Mar. 3, 1936 2,189,580Hewlett Feb. 6, 1940 FOREIGN PATENTS Number Country Date 40 68,188Sweden Oct. 8, 1929

